Reading or changing settings can be done using an implementation of the SettingsRepositoryInterface.
Because Flarum uses Laravel's service container (or IoC container), you don't need to worry about where to obtain such a repository, or how to instantiate one.
Instead, you can rely on the container to instantiate your class and inject the correct dependencies.

Each Form-group should contain an individual input. The bidi property on the input corresponds to the setting on the Settings table that the input will modify. It will be automatically propagated with the current settings value, and it will change the value once "Save Settings" button is clicked.

Oh? What's this? When you click "Save Settings" the (settings) Saved event is dispatched? How cool!